Within the field of telecommunications, there is a constantly increasing demand for higher data rates and capacity. In order to improve data rates in an existing cellular network, it has been proposed to deploy low power base stations at locations, or areas, where high data rates are desired. To increase capacity, low power base stations are useful in areas with high traffic density or where macro base stations can not provide sufficient. In this manner, the low power base station will be located within a grid of macro base stations. The low power base stations, such as micro and/or pico base stations and the like, typically have lower transmit power than the macro base stations in the grid. A deployment with low power base stations within a grid of macro base station is referred to as a “heterogeneous network” or “heterogeneous deployment”. Moreover, a layer of low power base stations is referred to as a micro/pico layer and the grid is referred to as macro layer, or macro layer grid.
Within the Third Generation Partnership Project Long Term Evolution release 9 (3GPP LTE), cell selection is based on power of reference symbols measured by a user equipment (UE). This power relates to the channel conditions the user equipment is experiencing. When the user equipment reports that it has better channel conditions to a neighboring cell, i.e. another cell than its serving cell, a handover to the neighboring cell is performed. The reference symbols are transmitted by a base station of the cell and a metric determined based on the reference symbol is commonly referred to as Reference Symbol Received Power (RSRP).
A known LTE system comprises a low power base station and a macro base station. A first coverage area of the low power base station is determined by measuring the power of reference symbols transmitted by the low power base station and comparing the measured power to a power level, measured on reference symbols transmitted by the macro base station. Since the transmit power of the macro base station is greater than the transmit power of the low power base station, a second coverage area of the macro base station is larger than the first coverage area of the low power base station. Thus, the low power base station may serve few user equipments, provided that there is a constant number of user equipments per unit area. In scenarios where the low power base station is located in a hotspot with a large number of user equipments, the low power base station may serve a significant number of user equipments.
It is important to offload the macro base stations by allowing the low power base stations to serve a significant share of the user equipments. If the macro base station is too heavily loaded (i.e. not offloaded enough), only user equipments served by low power base stations are able to reach high data rates. If fewer user equipments have to share resources of the macro layer, those user equipments can get higher data rates as well and more uniform data rates in the system can be achieved.
In many scenarios, it is expected that the low power base station is located in a hot spot such that user equipments in the hot spot are located closer to the low power base station than to the macro base station. A user equipment located near the low power base station can measure the power of the reference symbols from both the low power base station and the macro base station. The measured power of reference symbols from the low power base station may be higher than the measured power of reference symbols from the macro base station although the macro base station transmits reference symbols with higher power. As an example, this may happen when difference in pathloss (PL) between the low power base station and the macro base station is large. Pathloss is distance dependent. Therefore, user equipments close to the low power base station and far from the macro base station are often served by the low power base station.
In some releases of LTE, for example release 11 and higher, it is expected to be possible to extend the range of a cell of a low power base station by using a cell specific cell selection offset. By increasing this offset, the low power base station may be able to serve a larger number of user equipments.
In order to improve performance in the system, it has been proposed to apply a cell selection offset based on path loss. In this manner, the cell selection offset completely compensates for the difference in transmit power between the low power base stations and the macro base stations. Since all user equipments have similar transmit power, a cell selection offset based on path loss can be used in the uplink (UL) with reasonable success. With path loss based cell selection offset, the user equipment will be connected to the base station to which it has the best path loss conditions. As a result, the user equipment will experience an improved channel quality, interference will be reduced, and the load will be distributed more evenly between the macro base station and the low power base station. Distribution of load in the system may be referred to as load balancing. Hence, in the uplink the user equipments in an extended range of the low power base stations can get high data rates. “In an extended range” means herein an additional area that is served, i.e. covered, by the low power base station when a cell selection offset being greater than zero is applied, but was not served by the low power base station when the offset being equal to zero was applied.
Moreover, it has been proposed to increase the transmit power in the low power base stations. As a result, a coverage area of the cell of the low power base station is enlarged. As explained above, a higher transmit power yields a larger coverage area. However, in some scenarios, it may not be desired to increase the transmit power because the cost of such a low power base station with increased transmit power is expected to be higher than desired. Also, the size of the coverage area may become too large.
However, in the downlink (DL) the user equipments in the extended range may suffer from high interference from a neighboring macro base station. Known Inter-cell interference coordination (ICIC) schemes can be used to protect a data channel, but is not sufficient for control channels in the downlink. Therefore, in the case of range extension, or range expansion, it may be necessary to apply evolved ICIC based on Time Division Multiplexing (TDM) to protect downlink control channels. An evolved ICIC scheme may mute (or decrease) the transmit power of the macro base station according to a time schedule.
Consider a scenario, in which the macro layer is sufficiently offloaded by a low power base station thanks to a cell selection offset greater than zero, but a great number of user equipments is still served by the macro layer and only a few user equipments are served by the low power base station. In such scenario, muting of the macro layer will deprive the great number of user equipments, connected to macro base stations in the macro layer, from their resources in order to enable transmission to the few user equipments in the extended range of a low power base station. Since the great number of user equipments is deprived from their resources, performance for these user equipments may decrease even though the macro layer is sufficiently offloaded. As a result, there may be a decrease in performance for the system as a whole. Thus, in this scenario, cell range extension, or cell selection offset, does not provide a gain in terms of overall downlink system throughput. Therefore, the use of cell selection offset needs improvements.